1. Field of Invention
The present invention is directed to twisted pair cables, particularly those having twist lays, insulation thicknesses, insulation materials, and performance variables, such as characteristic impedance, that are optimized to achieve low skew.
2. Discussion of Related Art
High performance electrical cables are often used to transmit electrical signals between devices or components of a network. These cables typically include several pairs of insulated conductors twisted together, generally in a double-helix pattern about a longitudinal axis. Such an arrangement of insulated conductors, referred to herein as “twisted pairs,” facilitates forming a balanced transmission line for data communications. One or more twisted pairs may subsequently be bundled and/or bound together to form a data communication cable.
Modern communication cables must meet electrical performance characteristics required for transmission at high frequencies. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards which specify specific categories of performance for cable impedance, attenuation, skew and crosstalk isolation. For example, one standard for crosstalk or, in particular, crosstalk isolation, is TIA/EIA-568-A, wherein a category 5 cable is required to have 38 dB of isolation between the twisted pairs at 100 MHz and a category 6 cable is required to have 42 dB of isolation between the twisted pairs at 100 MHz. Various cable design techniques have been used to date in order to try to reduce crosstalk and to attempt to meet the industry standards. In addition, if cables are to be used in plenum, they must pass the Underwriter's Laboratory Standard 910 test, commonly referred to as the Steiner Tunnel test.
These specifications and requirements limit the selection of insulation materials that may be used in communication cables. Preferred insulation materials have been fluoropolymers because these materials provide certain desirable electronic characteristics, such as low signal attenuation and reduced signal phase delay. In addition, communication cables having insulation materials formed from fluoropolymers can pass the Steiner Tunnel test. Examples of fluoropolymer insulation materials used in communication cables include fluoroethylenepropylene (FEP), ethylenechlorotrifluoroethylene (ECTFE), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
However, fluoropolymer insulation materials also have disadvantages such as is relatively high cost and limited availability caused by the high demand for these materials. Therefore, several communication cables have been developed that replace some of the fluoropolymer insulation materials with certain non-fluoropolymer insulation materials. For example, U.S. Pat. No. 5,841,072 to Gagnon, herein incorporated by reference, discloses a twisted pair cable wherein each conductor of the twisted pair has a dual-layer insulation, the first (inner) layer being a foamed polyolefin including a flame retardant and the second (outer) layer being a fluoropolymer. In another example, a cable construction may comprise a mix of conductors, for example, with some conductors of the cable insulated with a single layer of fluoropolymer materials and others conductors in the same cable insulated with a single layer of polyolefin materials.
It is known that as the dielectric constant of an insulation material covering the conductors of a twisted pair decreases, the velocity of propagation of a signal traveling through the twisted pair of conductors increases and the phase delay added to the signal as it travels through the twisted pair decreases. In other words, the velocity of propagation of the signal through the twisted pair of conductors is inversely proportional to the dielectric constant of the insulation material and the added phase delay is proportional to the dielectric constant of the insulation material. Thus, using different insulation materials among conductor pairs within a cable may cause a variation in the phase delay added to the signals propagating through different ones of the conductors pairs. It is to be appreciated that for this specification the term “skew” is a difference in a phase delay added to the electrical signal for each of the plurality of twisted pairs of the communication cable. A skew may result from the insulation material covering one twisted pair of conductors being different than the insulation material covering another twisted pair of conductors of a communication cable.
In addition, in order to impedance match a cable to a load (e.g., a network component), a cable may be rated with a particular “characteristic impedance.” For example, many radio frequency (RF) components may have characteristic impedances of 50 or 100 Ohms and therefore, many high frequency cables may similarly be manufactured with a characteristic impedance of 50 or 100 Ohms so as to facilitate connecting of different RF loads. The characteristic impedance of the cable may generally be determined based on a composite of the individual nominal impedances of each of the twisted pairs making up the cable. The nominal impedance of a twisted pair may be related to several parameters including the diameter of the wires of the twisted pairs making up the cable, the center-to-center distance between the conductors of the twisted pairs, which may in turn depend on the thickness of the insulating layers surrounding the wires, and the dielectric constant of the material used to form the insulating layers.
In conventional manufacturing, it is generally considered more beneficial to design and manufacture twisted pairs to achieve as close to the specified characteristic impedance of the cable as possible, generally within plus or minus 2 Ohms. The primary reason for this is to take into account impedance variations that may occur during manufacture of the twisted pairs and the cable. The further away from the specified characteristic impedance a particular twisted pair is, the more likely a momentary deviation from the specified characteristic impedance the input impedance of at any particular frequency due to impedance roughness will exceed limits for both input impedance and return loss of the cable.
Many of the same parameters of a twisted pair affect both the characteristic impedance and the skew of a twisted pair cable. Therefore, there needs to be a balance or trade-off created between these parameters for the cable to meet all specified performance requirements, such as return loss, skew and crosstalk.